Project Summary Cell-to-cell communication is crucial for growth and development as well as stress adaption in multicellular organisms. Symplastic transport of signaling molecules between plant cells is mediated by membrane-lined channels termed plasmodesmata (PD), whereas gap junctions play similar roles in mammals. Although pathogenic bacteria infect multicellular organisms, it is unclear whether pathogenic bacteria exploit host cell-to-cell communication system to promote bacterial multiplication. To study host- pathogen interactions, I explore Arabidopsis-Pseudomonas syringae pv. tomato (Pst) DC3000 pathosystem. Typically, almost all phytopathogenic bacteria are extracellular pathogen, including Pst DC3000. Bacterial pathogens are recognized by plants via pattern recognition receptors (PRRs) on the plasma membrane (PM) and nucleotide-binding site-leucine-rich repeat (NBS-LRR)-type receptors inside the cell, resulting in defense activation. To overcome plant defense, Pst DC3000 injects 36 virulence effector proteins into host cells to exert their pathogenic activities. Using live cell imaging, I discovered that Pst DC3000 effector protein HopO1-1, an active ADP-ribosyltransferase, is targeted to the PD in Arabidopsis. Ectopic expression of HopO1-1 in Arabidopsis leads to an increase in PD-dependent molecular flux between plant cells. HopO1-1 is physically associated with Arabidopsis PD-located proteins (PDLP5 and PDLP7), in which expression of PDLP5 is upregulated by P. syringae infection and involved in plant innate immunity. To my knowledge, HopO1-1 is the first bacterial effector to be identified as a PD-targeted protein. The finding raises the exciting possibility that bacterial pathogens deliver effectors such as HopO1-1 to modulate host PD function, presumably by ADP-ribosylating PDLP5/7, to facilitate bacterial spread and multiplication. The mentored phase of this proposed work will be conducted at Michigan State University under the guidance of Dr. Sheng Yang He. During this phase, I plan to (i) characterize the mode of action of HopO1-1 in modulating the host PD, (ii) demonstrate the PD-dependent trafficking of bacterial effectors from the infected cells to the neighboring cells, and (iii) establish systems in studying the dynamic host-microbe interactions at the initial infection sites. Dr. He will provide specific training in microbiology and plant-microbe interactions as well as guidance toward becoming an independent researcher. In the independent phase, I plan to study (i) the effect of HopO1-1 in regulating PD aperture by controlling plant callose homeostasis, (ii) the effect of HopO1-1 in the movement of plant signaling molecules, and (iii) the functional role of HopO1-1 on creating bacterial infection zones to establish the initial colonization in host tissues. The proposed project has potential in carving out a new research topic in bacterial pathogenesis and establishing new experimental protocols in the study of cell-autonomous vs. non-cell-autonomous effects in host-bacterial interactions.